GENERATION MEAN ANALYSIS OF RESISTANCE TO RACE-3 OF SOYBEAN CYST-NEMATODE

The soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is a se rious pest of soybean [(Glycine mar (L.) Merr.]. Most genetic studies of resistance in soybean to SCN have used qualitative models, although continuous distributions of phenotypes from zero to many cysts have b een observed. We investigated the mode of inheritance for resistance t o Race 3 of SCN by weighted least squares regression analysis of gener ation means for genetic models having additive and dominance parameter s. Reaction to SCN was determined by counting the white females presen t on plant roots 30 d after inoculation. Four crosses, including one r eciprocal, were made between resistant and susceptible genotypes. The ancestry of the resistant genotypes ('A20', 'Jack', and 'Cordell') tra ces to different sources of SCN resistance. Six generation means (P-1, P-2, F-1, F-2, BCP1, and BCP2) in three of the crosses, and four in o ne (P-1, P-2, F-1, F-2), were used to estimate the genetic effects con tained in the means. The data were transformed for analysis because th ey were not normally distributed. The results indicated that an additi ve genetic model was sufficient to explain most of the genetic variati on for resistance to Race 3 in each cross. Analysis of the pooled data from all crosses, however, indicated the presence of dominance effect s as well. Recovery of parental phenotypes in the F,and backcross popu lations indicated that no more than four genes were involved in the in heritance of the trait. Estimates for broad sense heritabilities on si ngle plant basis ranged from 0.48 to 0.81. The relatively high heritab ility and additive genetic control indicate that breeders should be ab le to select genotypes with an intermediate level of resistance.